Plastic receptacles made of synthetic resin such as polypropylene are conveniently used as lightweight and cost-effective receptacles for foods. However, the plastic receptacles, though very convenient, have a problem in destruction by fire. That is to say, the combustion calorie of synthetic resin is much larger than those of paper and the like and hence it generates a great amount of combustion heat for its bulk. Accordingly, temperature in the incinerator rises very high, which may damage the incinerator or reduce its life time.
For the reasons above, demands for paper receptacles with low combustion calorie are increasing these days, as substitutes for the plastic receptacles. The table below shows a comparison of combustion calories between a paper receptacle and a plastic receptacle with the same bulk.
TABLE 1 ______________________________________ Plastic Paper Receptacle Receptacle ______________________________________ Capacity 150 ml the same Size 110 mm square .times. the same 27 mm high Material Area 220 cm.sup.2 the same Material Three-layers polypropylene Composition structure of: sheet (500 .mu.m) polymethyl pentene (22 .mu.m), paperboard (390 .mu.m), vinyl resin coat (2 .mu.m) Combustion 33 Kcal/piece 110 Kcal/piece Cal. ______________________________________
Values of physical properties of materials;
______________________________________ Specific Combustion heating gravity (g/cc) value (KCal/Kg) ______________________________________ Paperboard 0.75 4,400 polymethyl 0.83 11,000 pentene Vinyl resin 1.40 4,500 Polypropylene 0.91 11,000 ______________________________________
As can be clearly seen from the comparison above, the heating value of combustion of a paper receptacle is about one-third, or smaller, of that of a plastic one with the same bulk capacity.
FIG. 20 is a perspective view of a conventional rectangular paper receptacle with a curled periphery.
Referring to the figure, the rectangular receptacle has a bottom 123, peripheral walls 119 and 121 turned upwardly at a certain angle from the four sides of the bottom 123, peripheral corners 125 where the peripheral walls 119 and the peripheral walls 121 are connected, a flange 126 formed in the horizontal direction at the upper ends of the peripheral walls 119, 121 and the peripheral corners 125, and a curling 127 formed around the outer periphery of the flange 126.
FIG. 21 is a diagram showing the appearance of a blanked paperboard material for forming the receptacle shown in FIG. 20.
Referring to the figure, the paperboard material 101 is composed of a rectangular sheet material with four rounded corners. The dotted line shows the boundary corresponding to the bottom 123 of the formed receptacle. The parts located above and below it correspond to the peripheral walls 121, whose peripheries form peripheral straight portions 103. The parts located on the right and left sides of the bottom 123 correspond to the peripheral walls 119, whose peripheries form peripheral straight portions 105. As the parts corresponding to the peripheral corners 125, A score lines 117 are formed in the areas A to the outer periphery on radii extending from the central positions of the curved portions, 115a and 115b which are located in the bottom 123. The edges of the peripheral corners 125 form the curved portions 109 like circular arcs around the center positions 115a and 115b of the curved portions. In this diagram, the areas A are subjected to drawing in die forming or the like. The corners of the dotted line defining the bottom 123 are defined by the arcs of concentric circles with respect to the curved portions 109 around the centers 115a and 115b of the curved portions.
FIG. 22 is a process diagram schematically showing, in sections, the process of forming the paper receptacle of FIG. 20 with a forming machine using the paperboard material of FIG. 21.
The paperboard material 101 shown in (1) in the diagram is pressed between die members of the forming machine. Then the peripheral walls 119, 121 and 125 of the formed receptacle are formed as shown in (2), whose margins form the flat portion 129.
Next, as shown in (3), the flat portion 129 is formed into the horizontal flange portion 126 and an upstanding portion 133 for curling. Then as shown in (4), the upstanding portion 133 is curled and thus the curling 127 is formed around the entire periphery.
As stated above, the conventional rectangular paper receptacle with a curled periphery is formed from a sheet of paperboard material. Although a rectangular paper receptacle has been described as an example, circular receptacles are formed in the basically same way. In this case, the score lines are formed around the entire periphery of the paperboard material.
The conventional paper receptacle described above, having its entire periphery curled, is not sufficient as a receptacle when it is used as a receptacle with a cover or with protection film heat-sealed to protect its contents.
FIG. 23 is a cross-sectional view of the rectangular paper receptacle of FIG. 20 provided with a cover plate.
Referring to the diagram, the periphery 151 of the cover 150 is bent down to fit around the periphery of the curling 127 of the paper receptacle. Accordingly, increasing strength of the fit of the cover requires an increase of strength of the curling 127. As shown in FIG. 21, however, the score lines 117 are formed to the outer edge of the paperboard material 101, or to the part where the curling 127 is formed, and therefore the outer ends of the score lines 117 are intactly curled, resulting in reduction of the strength of the curling 127.
FIG. 24 is an end view of the main part of the rectangular paper receptacle of FIG. 20 with protection film sealed with heat.
Referring to the diagram, synthetic resin film 151, composed of a composite material of synthetic resin films of polypropylene, polyethylene, or the like, or of a composite material of paper or aluminum foil and a synthetic resin film or a heat-sealed material, is heat-sealed on the top surface of the flange 126 of the rectangular receptacle. To provide good heat sealing, it is preferable to form the top surface of the flange 126 smooth. However, particularly in the case of a rectangular receptacle, the corner portions are apt to be corrugated in forming. This degrades the smoothness of the flange 126, which may cause inferior heat sealing.
Furthermore, recently, paper receptacles are used as easy heat-resisting receptacles, heated in an oven or the like together with foods contained therein. In such a case, the paper receptacle is formed by using a paperboard provided with heat-resisting resin coating or heat-resisting resin film (density: 0.85-0.90, permeability (by the testing method based on the English version of JIS P8117): 250-600 sec) on its inner side.
This publication specifies the testing method for the air permeability of paper and paperboard that permits 100 ml air to pass through an area of 645 mm.sup.2 in 2 to 1800 seconds. Such materials as crepe and corrugated papers of which clamping down cannot shut out its surface and edge leakage, are excluded from the application of this standard.
The testing device is divided into two types, Types A and B, consisting of an outer cylinder partly filled with oil and an inner cylinder which can freely slide in the outer cylinder and having an open or closed top. In Type B, the cylinder is of concentric double cylinder construction, having an open top, containing the oil in itself, and the inner cylinder forms an air passage reaching the lower clamping plate. Air pressure for the test is provided by the mass of the inner cylinder.
The testing device is of a construction capable of applying an air pressure onto the test piece held between the clamping plates having a circular orifice of 28.6.+-.0.1 mm in diameter. The clamping plates may form the top of the inner cylinder (in Type A) or may be mounted in the base of the testing device (in Type B). (The latter construction is disclosed as being preferable.) An elastic gasket is attached to the clamping plate on the side exposed to air pressure, and the test piece is held in contact with the gasket when clamped for the test.
The gasket consists of a thin, elastic, oil-resistant, nonoxidizing material having a smooth surface and capable of preventing air from leaking through the test piece and the clamping plate. An oil-resistant rubber, such as grade S.T. Thiokol gasket of 0.77 mm in thickness, and of 50 to 60 in Durometer hardness, is disclosed as being a satisfactory gasket material. The inside diameter of the gasket is 28.6 mm and the outside diameter 34.9 mm. The bolt holes in the gasket are centered exactly to those in the clamping plate, and, in order to align and protect the gasket in use, it is cemented with shellac into a groove machined in the clamping plate. This groove is concentric with the aperture in the opposite orifice plate, and 28.4 mm in inside diameter, 35.2 mm in outside diameter and 0.5 mm in depth for convenience in inserting and attaching the gasket. The outer cylinder is 254 mm high and has an internal diameter of 82.5 mm, and marked with a level line at 127 mm from the inner bottom.
The outer cylinder is equipped with four bars, each 190 mm in length, 2.4 mm in width and 2.4 mm in thickness, on the inner surface to act as guide tracks for the inner cylinder. The inner cylinder is graduated in units of 50 ml, and has a total range of 350 ml. It is 254 mm high, and has an external diameter of 76.2 mm, an internal diameter of 74 mm and a mass of 567.+-.1.0 g. Or, the inner cylinder may be graduated in units of 25 ml for the first 100 ml and have a range of 400 ml.
The oil used in the testing device is a lubricating oil having 60 to 70 seconds Saybolt Universal viscosity at 37.8.degree. C. {10 to 13 mm.sup.2 /s} and a flash point of not less than 135.degree. C.
The publication discloses that a light spindle oil is suitable for this purpose. Oil is used in preference to water, because it does not affect the moisture content of the sample nor does it affect the aluminum inner cylinder. The oil does not contain any essential oil or easily volatile oil.
The device is tested for air leakage by clamping a thin piece of smooth, hard-surface airtight material, such as metal foil or cellophane, clamped between the orifice plates. The leakage should not exceed 50 ml in 5 h.
The test piece is approximately 50.times.130 mm in size. Test pieces of not less than 50.times.50 mm may be used in the device having the clamp in the base.
Test pieces are conditioned prior to testing.
The testing device is placed on a level surface so that the inner cylinder becomes vertical. The outer cylinder is filled with oil to the level line of 127 mm depth marked on the inner surface of the cylinder.
If Type A device having the clamp in the top of the inner cylinder is used, the inner cylinder is raised, held in a raised position with one hand, the test piece is clamped between clamping plates, then the cylinder is lowered, and allowed to float on the oil.
If Type B device is used, the inner cylinder is taken out, the test piece is clamped, the inner cylinder is inserted into the outer cylinder, the inner cylinder is gradually lowered, and allowed to float on the oil.
When the device having the clamp in the base is used, first the inner cylinder is raised until its top rim is supported by the catch, the test piece is clamped between the clamping plates, then the inner cylinder is gently lowered until it floats. When the steady movement of the inner cylinder has been attained, using a stopwatch or other timing device, the number of seconds required for the graduations from 0 to 100 ml to pass the rim of the outer cylinder are measured. For very resistant papers, the reading may be taken at the end of first 50 ml graduation, and the results doubled. For porous papers, the number of seconds for 100 ml or over may be read, and converted to the 100 ml standard volume.
The test for at least five test pieces for each top side and back side is performed, and the average of the results is taken. However, for heterogeneous papers, the test is performed for not less than 10 test pieces, and the average is taken by discarding extraordinary values.
In clamping the test piece, the inner cylinder is suspended with one hand, and the nuts alternately fastened so that the pressures on both sides become equal to each other. Care should be taken not to excessively fasten the nut on either side alone, or it can cause air leakage through the clamping plate and the test piece.
The average number of seconds required for the displacement of 100 ml of air through the paper of an area of 645 mm.sup.2 is taken as the air permeability, and the value which is rounded to two significant digits is reported.
In this case, when the bottom of the paper receptacle is heated (e.g., when cooked at atmospheric temperatures of 200-250.degree. C. for 10-20 minutes), then the heat-resisting coating or the heat-resisting film may be lifted and peel off from the paperboard.